The present invention relates to new anhydrous crystalline forms of gabapentin prepared from gabapentin monohydrate.
Gabapentin is a generic name used to identify the chemical compound (1-aminomethyl)-1-cyclohexaneacetic acid 
It is useful in therapy of certain cerebral disorders such as certain forms of epilepsy, faintness attacks, hypokinesia, and cranial traumas. U.S. Pat. Nos. 4,024,175 and 4,087,544 cover the compound and its uses. They also disclose an acid salt, i.e., gabapentin hydrochloride hydrate in a ratio of 4:4:1 and a sodium salt of gabapentin hydrate at a ratio of 2:1. U.S. Pat. No. 4,894,476 describes gabapentin monohydrate and a process for producing it. These patents are incorporated by reference.
The present invention provides new crystalline forms of gabapentin, dehydrate Form A and dehydrate Form B. These new crystalline forms are prepared from gabapentin monohydrate. The gabapentin monohydrate is dehydrated in an environment in which the water activity is maintained below about 0.8 to about 0.9, or heated at a temperature from about 50-175xc2x0 C. to form the crystalline gabapentin dehydrate Form A. This crystalline form is then converted on standing at ambient temperatures to a more stable crystalline form of gabapentin, dehydrate Form B.
The present invention provides further process improvements to provide pure crystalline forms of the therapeutic agent, gabapentin. The present process eliminates the need of using organic solvents such as methanol described earlier in methods of producing gabapentin and its monohydrate. The present process offers faster manufacturing processing, better safety, less solvent disposal and less loss of yield upon recrystallization.